1. Field of the Invention
The present invention relates to a tensioner used to apply a proper tension to a timing belt, timing chain or the like of an automobile engine.
2. Description of the Related Art
Tensioners are widely used for a timing belt or a timing chain which transmits rotation from a crankshaft of an engine to camshafts, in order to prevent the timing belt or the timing chain from vibrating when the vehicle is running, and to keep a proper tension of the timing belt or chain.
FIG. 8 shows a chain transmitting device of an engine in which such a tensioner is incorporated. The tensioner 310 is mounted on the engine adjacent to a slack side of a chain 320 which is stretched between a driving sprocket 314 driven to rotate by a crankshaft 312 of the engine and a driven sprocket 318 fixed on a camshaft 316 of the engine.
The tensioner 310 has a plunger 324 slidably received for reciprocation in a tensioner housing 322 with one end portion protruding from a front surface of the housing 322. The plunger 324 forces the back surface of a tensioner lever 328 at a position near a free end of the tensioner lever 328 which is pivotally supported at the opposite end a pivot shaft 326 secured to the engine body, so that a shoe surface 330 of the tensioner lever 328 is urged into sliding contact with the slack side of the chain 320 thereby to apply a tension to the chain 320.
Japanese Patent Laid-open Publication No. 2000-240744 discloses an example of such tensioners. FIG. 1 of this publication shows a plunger installed in a tensioner housing freely slidably, with a high pressure oil chamber being formed by the housing and the inside of the plunger. The plunger is installed so as to be urged in the protruding direction by a plunger spring, and has a plug mounted at a front end of the plunger.
Provided between the housing and the high pressure oil chamber is a check valve mechanism which has a check ball that allows oil to flow into the high pressure oil chamber and prevents the oil from flowing out from the high pressure oil chamber. The high pressure oil chamber is filled with oil which is continuously supplied from an oil source through an oil supply passage and the ball check valve mechanism.
An auxiliary oil chamber communicating with the high pressure oil chamber via an oil passage is formed in the plunger. The auxiliary oil chamber has a valve installed therein which is urged in the plunging direction by a valve spring, while the plunger has a discharge port which opens when the valve slides against the force of the valve spring, thereby constituting a relief valve mechanism.
In the known tensioner described above, oil pressure in the high pressure oil chamber increases when the chain runs and the plunger receives a load applied by the chain in the direction opposite to the urging direction of the plunger spring. At this time, hydraulic pressure is applied to the auxiliary oil chamber where the valve of the relief valve mechanism makes contact freely slidably to the inside thereof, so that the valve retracts against the urging force of the valve spring. When the oil pressure increases further, the valve retracts further and eventually opens the discharge port which communicates with the outside, so that the oil is discharged from the auxiliary oil chamber so as to mitigate the excessive pressure in the high pressure oil chamber.
In the conventional tensioner described above, maximum displacement of the valve spring corresponds to the distance traveled before the valve and the plug make close contact with each other. While it is not necessary to displace the valve and the valve spring after the discharge port of the valve has been opened, though in case the oil pressure in the high pressure oil chamber experiences an instantaneous increase, the valve spring experiences a displacement due to inertia of the valve after the discharge port of the valve has been opened.
By the displacement of the plunger in the protruding direction, wire of the valve spring is subject to shear stress. As will be understood from the relationship between the S-N curve and the fatigue limit, the valve spring breaks due to fatigue when the wire is subjected to shear stresses of intensities higher than a certain level repetitively. As a result, there has been a problem that the relief valve mechanism fails to function.
An object of the present invention is to provide a tensioner having a relief valve mechanism wherein the maximum displacement of the valve spring of the relief valve mechanism is restrained within a range which is necessary and sufficient, and the stress acting in the wire of the valve spring is restrained within the fatigue limit.
To achieve the foregoing object, according to the present invention, there is provided a tensioner comprising: a housing; a plunger slidably mounted in the housing with one end protruding outward of the housing under the force of a plunger spring disposed inside the housing, there being a high pressure oil chamber defined between the housing and the plunger; a check valve mechanism having a check ball which allows oil to flow from the outside of the housing into the high pressure oil chamber and prevents the oil from flowing out from the high pressure oil chamber; and a relief valve mechanism having an auxiliary oil chamber which communicates with the high pressure oil chamber, a valve displaceable in a direction to enlarge and reduce the volume of the auxiliary oil chamber, a discharge port which allows the oil to flow out from the auxiliary oil chamber when the valve has moved over a predetermined stroke in the direction to enlarge the volume of the auxiliary oil chamber, and a valve spring which urges the valve in the direction to reduce the volume of the auxiliary oil chamber, the valve spring being a compression coil spring formed from a wire having a fatigue limit. The tensioner further has a protection means associated with the relieve valve mechanism for limiting a maximum shear stress acting in the wire of the valve spring below the fatigue limit when the valve has been displaced so as to open the discharge port.
In the tensioner described above, oil pressure in the high pressure oil chamber increases when a chain used with the tensioner runs and the plunger receives a load applied by the chain in the direction opposite to the urging direction of the plunger spring, since the check valve mechanism prevents the oil from flowing out from the high pressure oil chamber. At this time, oil pressure equal to that in the high pressure oil chamber is applied to the auxiliary oil chamber. When the force which is the product of the oil pressure and the sectional area of the auxiliary oil chamber becomes larger than the urging force of the valve spring, the valve starts to move in the backward or retracting direction and the valve spring begins displacement in the compressed direction, so that the impact on the plunger is suppressed. When the oil pressures in high pressure oil chamber and the auxiliary oil chamber increase, the valve retracts so as to open the discharge port which communicates with the outside, so that the oil is discharged from the auxiliary oil chamber thereby to diminish the excessive pressure in the high pressure oil chamber.
According to the invention, since the maximum shear stress acting in the wire of the valve spring is restrained by the protection means within the fatigue limit of the valve spring wire when the valve retracts to open the discharge port, the valve spring does not break even after having expanded and contracted repeatedly. This ensures that the relief valve mechanism remains functional.
In one preferred form of the present invention, the plunger has a spring retainer for retaining an end of the valve spring, and the protection means comprises a stopper surface formed on the spring retainer, the stopper surface being adapted to engage the valve when the valve has been displaced so as to open the discharge port.
The valve may have a flange-shaped enlarged head having an outside diameter larger than the outside diameter of the valve spring, the enlarged head being adapted to engage the stopper surface. The valve may further have a support boss projecting from the flange-shaped enlarged head and received in an axial hollow space of the valve spring being the compression coil spring. With this arrangement, it is possible to physically restrict the maximum displacement of the valve and the maximum stroke of the valve spring, thereby restricting the maximum shear stress acting in the valve spring wire less than, for example, the fatigue limit.